It’s a balancing act — the downside of having a moderator splitting off a thread is that the new thread can be difficult to read because it abruptly starts in the middle of a conversation. By starting a new thread intentionally, on the other hand, the initial poster can (hopefully) begin it with a clear summary of the new topic (even if it links to other topics).
See also this comment: Time limits for unfocused discourse threads? - #11 by Nathan_Boyer